With the conventional vehicle periphery monitoring system that generates a bird's-eye view image, generally an image photographed by an in-vehicle camera is projected onto a projection plane parallel with the road surface. Namely, with a projective transformation with positioning a virtual viewpoint perpendicularly upward, a bird'-eye view image from the immediately upward is generated. By causing this bird'-eye view image to be displayed on a monitor, a driver can effectively monitor a road surface condition in the vehicle periphery, in particular, an obstacle present on the road surface. However, as this in-vehicle camera employs a wide angle lens, in the bird'-eye view image obtained through the projective transformation, there occurs a greater distortion in the image in an area far from the camera than an area near the camera, which distortion makes it difficult for the driver to grasp the direction of the obstacle.
For instance, a three-dimensional object (a human, a car, an on-road 3D object such as a road cone for construction work, etc.) placed on the road surface and extending upward will appear in a distorted shape elongated in the photographing direction of each camera. In particular, in synthesis of bird's-eye view image segments generated from respective photographed images, if this synthesis is effected only by simple merger (blending) of mutually overlapped areas thereof, this will generate two images of the 3D object with elongations in the respective photographing directions, thus further deteriorating the visibility.
For alleviating the above problem, according to a vehicle periphery monitoring apparatus described in Patent Document 1, by multiple photographing means having mutually overlapped photographing areas, images of the periphery of a self vehicle are photographed. Then, through transformation of multiple photographed images thus obtained, a plurality of bird's-eye view images are generated and differences among these multiple bird's-eye view images are calculated. With this, in the overlapped areas, there are determined agreement areas where the images are in agreement and non-agreement areas wherein the images are not in agreement. Then, an output image is generated through synthesis of the agreement areas and the non-agreement areas by a plurality of techniques to be displayed for a driver. By generating the output image synthesis of the agreement areas and the non-agreement areas by a plurality of techniques, an attempt is made to resolve the poor visibility of the image in the overlapped area. However, in the bird's-eye view image thus generated, the boundary between the overlapped area and the non-overlapped area is not recognizable. Hence, it is difficult to judge with which camera an image of an object located in the overlapped area was photographed. As a result, there arises inconvenience of difficulty in grasping the position of the object (obstacle) appearing in the overlapped area.
Patent Document 2 discloses a vehicle periphery monitoring apparatus having a function of drawing a join masked pattern (indicated by a bold straight line in the illustration) at a join portion which is formed at the time of synthesis of two photographed images obtained by adjacent cameras and a function of generating an average image through alternate selection of mutually corresponding pixels for the pixels of overlapped areas of two photographed images.
Moreover, Patent Document 2 describes a further function of flashing ON/OFF this join masked pattern for calling the driver's attention when an obstacle detection sensor of ultrasonic type has detected an obstacle in the join portion. That is, as the join portion is masked as a dead spot in the photographed image, no bird's-eye view image is displayed of this join portion. Therefore, in case no obstacle detection sensor is provided, the driver cannot visually recognize an obstacle present in the join portion on the monitor screen. Further, even when an obstacle detection sensor is provided, since the arrangement provides only flashing on/off the join masked pattern, it is difficult for the driver to recognize the obstacle present in the join portion on the monitor screen.